Conventionally, a lead storage battery has been used for starting an engine of a vehicle and for a back-up power source. Among these usages, a lead storage battery for starting an engine functions to supply electricity to various electric and electronic devices mounted on vehicles, in addition to a cell motor for starting an engine. After starting an engine, a lead storage battery is charged by an alternator. An output voltage and an output current of the alternator are set so that SOC (state of charge) of the lead storage battery is maintained to be 90 to 100%.
In recent years, a demand for an improvement of a fuel-efficiency of a vehicle is increasing, in view of environmental conservation. For such a demand, a vehicle carrying a stop-and-go-system and a regenerative-braking-system has been considered, for example. In the stop-and-go-system, an engine is stopped while the vehicle is idling, and in the regenerative-braking-system, a kinetic energy of a vehicle at the time of deceleration is converted to an electric energy, and the electric energy is stored.
In a vehicle carrying the stop-and-go-system, the lead storage battery is not charged when the vehicle is stopped in an idle stop mode. The lead storage battery sometimes supplies electric power to devices mounted on the vehicle while in such a state. Thus, in comparison with a conventional lead storage battery for starting engines, SOC of the lead storage battery inevitably becomes low. In a vehicle carrying the regenerative-braking-system, SOC of the lead storage battery has to be controlled to be lower, to about 50 to 90%, since electric energy is stored by the lead storage battery at the time of regeneration (deceleration).
In any of these systems, charge and discharge (hereinafter referred to as charge/discharge) are repeated frequently with a lower SOC domain than ever. Further, based on an increase in a dark current accompanied with vehicle parts increasingly becoming electrically powered, a discharge of the lead storage battery advances while a vehicle is stopped for a long period of time, thereby leaving a possibility for an over discharge.
Therefore, for a lead storage battery to be used in vehicles carrying these systems, service life properties under a usage mode in which charge/discharge is repeated frequently with a lower SOC domain needs to be improved.
For deterioration factors of a lead storage battery under such usage mode, an insufficient charge due to decline in chargeability of the lead storage battery can be mentioned, mainly. Since a charge system of a vehicle is based on constant voltage control, when chargeability of a negative electrode plate is reduced, a potential of negative electrode decreases at an initial stage of charge and a voltage rapidly rises up to a preset voltage value, and a current decreases sooner. Thus, a sufficient amount of charged electricity of a lead storage battery can not be secured, thereby rendering the battery to be in an undercharged state.
For suppressing such deterioration, there has been proposed a method in which a lead alloy layer containing Sn and Sb is formed on a surface of a positive electrode grid of a Pb—Ca—Sn alloy, for example (Patent Document 1). The formation of such layer will suppress a deterioration of positive electrode active material and a formation of a passivated layer at an interface between a positive electrode active material and a positive electrode grid.
Also, a part of Sb which exists on a surface of a positive electrode grid dissolves in an electrolyte, and deposits on a negative electrode plate. The deposited Sb on a negative electrode active material will raise a charging potential of a negative electrode plate, and a charging voltage will lower down, thereby improving chargeability of a lead storage battery. As a result, deterioration of a lead storage battery due to insufficient charge during charge/discharge cycle is suppressed.
This method is very effective in an engine starting lead storage battery which is used while SOC is over 90%, and service life properties will drastically improve.
However, when a lead storage battery is to be used in a vehicle equipped with the above stop-and-go-system or the regenerative-braking-system, that is, when a lead storage battery is to be used in a mode in which charge/discharge is repeated under lower range of SOC, there was a problem in that a corrosion easily advances in a tab of negative electrode grid, while the chargeability can be secured. When the corrosion advances in a tab of negative electrode grid, current collecting efficiency of a negative electrode plate declines due to a decrease in a thickness of the tab, thereby shortening its service life.
The decrease in a thickness of the tab of negative electrode grid also weakens intensity of the tab, in addition to the decline in the current collecting efficiency. Especially, in batteries mounted on vehicles, since vibrations and impacts are constantly applied to the batteries while traveling, deformations of a tab of negative electrode grid cause a negative electrode plate to become out of position, leaving a possibility for an occurrence of an internal short-circuit by a negative electrode plate making contact with a positive electrode plate.
Conventionally, regarding the corrosion of a tab of negative electrode grid, it has been known that by an exposure of a negative electrode strap and a tab of negative electrode grid from an electrolyte to oxygen in air, a welded part of the strap and the tab is corroded to cause a disconnection. However, although a negative electrode strap and a tab of negative electrode grid are being immersed in the electrolyte, the tab of negative electrode grid is easily being corroded when Sb is deposited in a very small amount on a surface of the tab of negative electrode grid by dissolution of Sb included in a positive electrode grid, and in a positive electrode connecting member comprising a positive electrode strap, a positive electrode pole, and a positive electrode connecting body into an electrolyte.
In Patent Document 2, there has been proposed a lead storage battery in which a positive electrode grid, a positive electrode connecting member, a tab of negative electrode grid, and a negative electrode connecting member are formed from Pb or a Pb-alloy not including Sb, and a very small amount of Sb is included in either one of a negative electrode grid excluding a tab-part or a negative electrode active material layer to a degree that would not increase an amount of electrolyte loss. Based on such structure, it has been shown that chargeability and service life for a deep discharge of a battery are improved to a certain extent, by suppressing the dissolution of Sb existing in a positive electrode plate into an electrolyte and deposition of Sb on a tab of negative electrode grid.
However, the above lead storage battery as well had a problem in that under a usage mode where a charge/discharge is repeated frequently while SOC is in a low range, a tab of negative electrode-grid is corroded by the dissolution of Sb in a negative electrode grid excluding a tab part into an electrolyte, and by a deposition of Sb on a tab of negative electrode grid.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 03-37962    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-346888